Because of issues of cost and safety, lithiated metal phosphate materials are finding increasing utility as components of electrodes for electrochemical devices, and in particular as components of cathodes for rechargeable lithium-ion batteries. In the operation of such batteries, lithium ions are transferred, via an appropriate electrolyte, from the cathode to the anode during charging and from the anode to the cathode during discharge.
Lithiated metal phosphates have good thermal stability, low reactivity with electrolytes, and can be made from inexpensive and abundant raw materials. Lithiated metal phosphates have very good lithium ion transport and storage properties which allows for the manufacture of lithium ion batteries having large charge storage capability. However, these materials have relatively low electronic conductivity, and this factor limits their use in electrochemical devices.
The prior art has implemented various approaches in an effort to enhance the electronic conductivity of this class of materials. In one approach, particles of lithiated metal phosphates are coated with an electrically conductive material such as carbon. This does enhance the electronic conductivity of the materials; however, the carbon, while having good electronic conductivity, has poor storage and transport properties for lithium ions; hence, the presence of carbon decreases the specific electrochemical capacity and lowers the tap density of the electrode material. Another prior art approach to enhancing the electronic conductivity of these materials involves the use of dopants. In some instances, dopant metals were added to the M1 (lithium) site of the material; however, doped materials thus produced should impede lithium ion conduction by blockage of the conduction path. In addition, general skepticism has been expressed in the art regarding aliovalent doping of lithium iron phosphate materials insofar as it was believed that the olivine structure is unfavorable to such doping. In view of the foregoing, the art has generally looked to the use of composite materials such as carbon-coated materials for providing electrically and ionically conductive metal phosphate electrode materials.
As will be explained in detail hereinbelow, the present invention breaks with prior art approaches and provides a doped, lithiated metal phosphate material having good transport properties for both electrons and lithium ions. As such, the present invention provides for the manufacture of improved electrochemical devices, and in particular improved, rechargeable lithium ion batteries.